I never really was into having music in the background while I play RPGs. However it's something I've begun to dabble in. I don't go for making a specific playlist. Rather I started using soundtracks. I can typically just put it on a loop without having to worry about the music being a hindrance to play.
MMO soundtracks are pretty good sources for background music, and one of my favorites of late has been the Age of Conan Soundtrack. It just fits my Savage Worlds hack of Dark Sun very well. Just the right touch to give the session a little ambiance and not be too distracting.
Massively is a MMO news site that recently posted a gold mine of links for free MMO soundtracks. It's a bit of a pain to go through as many are individual tracks. However I think you can easily have a ton of tunes to mix and match for your game. The links likely cover a pretty wide spread of 'sci-fi' themed stuff to your typical fantasy music. Hope folks find this useful for their games.